1. Field of the Invention
The present invention is directed to a circuit for clamping black level reference signals contained in a video signal sequence of an optical line scanner which comprises a charge coupled device (CCD) sensor, using a video amplifier and an analog-to-digital converter, and a memory element controlled by the black reference signal for the controlled quantity that influences the video intensifier.
2. Description of the Prior Art
Image and line sensors preferably constructed in semiconductor technology, particularly sensors in integrated CCD circuit technology are employed in optical character and image pattern recognition. They usually supply video signals at their outputs that must still be appropriately conditioned in view of the application, the sensitivity and the setting to a defined black level. The setting to defined black level is frequently referred to as "black level clamping" in video technology. A standard, desired level for the signal corresponding to the "black" is zero volts. This black level differs greatly not only from type-to-type, but also from unit-to-unit and can also be additionally dependent on the temperature of the silicon chip.
It is therefore the desire for a circuit that set the output signal of CCD sensors to a defined DC voltage level, namely, particularly such that the signal level of the zero volts is assigned to the video signal "black". The problem to be solved here is inherently known from video technology because the video signal is frequently transmitted without a DC voltage level. A defined form, however, what is referred to as the "black shoulder" appears in the sequence of the video voltage signal and the video signal can usually be clamped thereto with a simple circuit composed of a series capacitor, a shunt resistor and a shunt diode. This circuit, however, has the disadvantage that the capacitor and the resistor form an RC element whose cut-off frequency must lie adequately for outside of the frequency band to be transmitted. Moreover, the circuit is generally too inaccurate because the threshold voltage of the clamping diode is dependent on the type of diode, on the unit employed and on the temperature and this threshold voltage enters on the black level.
Given CCD sensors wherein black reference picture elements are inserted into the video signal sequence at a defined location, the reference level is "filtered out" of the filter sequence, is stored and is subtracted from the current video signal. FIG. 1 illustrates the principle of such a circuit. It shows a CCD line sensor ZEIS intended for the reception of optical signals OS, a cycle sync signal SYNC and a CCD transport clock T from a time signal circuit ZSIG being supplied thereto. Black reference picture elements are inserted in the output signal of the CCD line sensors ZEIS at a defined location, these being taken out with the assistance of demultiplexers DEMUX clocked by the black level reference signal BR being held in a hold amplifier and being subtracted from a current video signal of the CCD sensor ZEIS in a video signal amplifier VID. Finally, the output signal of the video signal amplifier VID is supplied to an analog-to-digital converter ADC. Black level clamping in the digital signal domain would have required 8-bit conversion. So the analog-to digital converter ADC is not included into the clampling circuit and 6-bit-conversion is sufficient. However, the desired accuracy can be achieved only with a relatively high development and circuit expense because of the sources of error lying in the region of the analog demultiplexer DEMUX and of the video signal amplifier VID.